Transmission electron microscopy of Cd3As2 and eutectoid Cd3As2-As crystals

Abstract

Cadmium arsenide, Cd3As2, is a narrow band-gap semiconductor with forbidden energy gap of 0.14 eV. The electron density in as-grown n-type crystals is as high as 2 3 10 my3 [1] and electron mobilities have been reported from 0.3 to 2.0 m Vy1 sy1 at 300 K, while approaching 28 m Vy1 sy1 at 4.2 K [2, 3] making them suitable for photo detectors, thermal detectors, ultrasonic multipliers, and Hall generators [4, 5]. Hence, crystal and band structures remain of interest [6]. Its accepted crystal structure is tetragonal with a ˆ 1:267 nm, c ˆ 2:548 nm and space group I41cd [7]. This work examined the crystal structure of Cd3As2 and lattice defects using transmission electron microscopy (not reported on bulk material). Crystals were grown by sublimation under two sets of conditions, by transport within an evacuated silica ampoule in a temperature gradient with condensation occurring above the solid-solid phase transition of 578 8C [8], and in an argon gas ow with recondensation in a temperature region of 335 8C to 385 8C as described by Lovett [9]. Specimens for electron microscopy were prepared by grinding and polishing followed by argon ion etching. Intensities and d-values of diffraction peaks for Cd3As2 obtained by powder X-ray diffractometry are as reported by Steigmann and Goodyear [7], supporting a body-centred tetragonal with a unit cell of 16 cubes. In each cube, the As ions are arranged approximately cubic close-packed (f.c.c.), and the Cd ions are tetrahedrally co-ordinated. Certain of the cadmium ions are shifted from their idealized positions by approximately 0.02 nm, and others by 0.025 nm. The arsenic ions are also displaced and As±As bond lengths vary from 0.448 to 0.47 nm. These ionic displacements lead to violation of extinction conditions in this structure. A series of electron diffraction patterns of Cd3As2 were obtained using a JEM-200CX transmission electron microscope (TEM). A reciprocal lattice cell, as shown in Fig. 1, was reconstructed using these electron diffraction patterns. The lattice parameters thus derived are a  1:26(7) nm, and c  2:54(8) nm. Part of the series of diffraction patterns is shown in Fig. 2. These diffraction patterns can be indexed unambiguously based on the lattice parameters obtained above. The zone axes for each diffraction pattern are [0 0 1], [2 0 1], [1 0 0], [2 1 0], and [1 1 1], respectively. However, there are additional diffractions which are not observed in the X-ray powder diffraction spectrum. These diffractions are forbidden re ections. Their occurrence in electron diffraction patterns is because of the displacements of cadmium and arsenic ions from their exact positions in the unit cell, and because of double diffraction. For example, in the pattern of Fig. 2c, there is a column of diffraction spots between 0 0 0 and 0 4 0 and three rows of diffraction spots between 0 0 0 and 0 0 8, and all the alternative columns and rows which are parallel to these. These are forbidden re ections in this structure. The faint diffraction spot of (0 0 4) in Fig. 2c becomes a strong re ection in Fig. 2d due to double diffraction of (1 2 1)‡ (1 2 3) ˆ (0 0 4). All the faint spots in Fig. 2e are forbidden diffractions, but their occurrences are due to ionic displacements, and not because of double re ections. The appearances of forbidden re ections due to ionic displacements depend also on the specimen thickness. Fig. 2f is a diffraction pattern with the electron beam parallel to [1 1 1], but is taken from a thinner specimen and is more populated with forbidden diffractions such as (h h 0) or (0 k k). Certain patterns from samples prepared under the second set of growth conditions exhibited different symmetry. Energy dispersive X-ray analysis of these distinguishable crystalline regions showed them to be arsenic. A eutectoid mixture of Cd3As2 and As with up to ,5 at % As was observed by Carpenter et al. [10] in ®lms of evaporated Cd3As2 as prepared using a pulsed-laser technique. In the present work, the proportion of As in the (Cd3As2 ±As) eutectoid was less than ,0.1 at %. Electron diffraction patterns of these arsenic phases indicated that their structure is orthorhombic with parameters a ˆ 0:71(3) nm, b ˆ 0:73(3) nm, c ˆ 1:25(1) nm and with space group Pnnn, rather than hexagonal. Electron diffraction patterns with zone axes [1 0 0] and [1 3 0] are shown in Fig. 3. The new arsenic unit cell is possibly due to the presence in solution of low levels of cadmium below the detection level of EDS X-ray analyser (Link Systems, 860 series II). Using bright-®eld and dark-®eld imaging, structural defects were observed within the Cd3As2 crystals and within the new phase of arsenic.